Therapeutic monoclonal antibodies have been used successfully as treatments for a variety of diseases. The relatively long in vivo half life of antibodies is mediated at least in part by the interaction of the Fc region of the antibody with the neonatal Fc receptor (FcRn). See, e.g., Ghetie et al. (1996), Eur. J. Immunol. 26: 690-96. FcRn binds to the Fc region of IgG antibodies with nanomolar affinity (KD≈100 nM) at a pH of less than or equal to 6.0, but does not bind at the pH of blood, i.e., about pH 7.4. Tesar and Björkman (2001), Curr. Opin. Struct. Biol. 20(2): 226-233. Upon internalization of an antibody by a cell, for example by pinocytosis, an IgG can be bound by FcRn in the acidic environment of an endosome. Id. When bound by FcRn within the endosome, the IgG will be directed back to the cell surface, as opposed to entering a default catabolic pathway within the endosome. Id. In the generally physiologic pH environment at the cell surface, the IgG can dissociate from FcRn and re-enter the circulation. This process allows the antibody to return to the circulation following internalization within a cell, as opposed to being degraded in the cell within an endosome.
There is a need in the art for therapeutic antibodies with increased in vivo half lives so as to decrease dosing amounts and/or frequencies. Such antibodies are advantageous because of increased patient convenience, and therefore also possibly increased patient compliance, and/or decreased cost. In the current cost-conscious health care environment, cost can be a determining factor in the practical utility of a therapeutic product.